Lu:nd:yag laser system and material

ABSTRACT

A laser system having an elliptical laser cavity with a laser rod and a broadband pump source mounted in the laser cavity. The laser rod is a co-doped Lu:Nd:YAG crystal having a Lu/Nd atomic ratio in the range from 2.0 to 2.4 distributed in the YAG host crystal. Because the laser rod absorbs a relatively small amount of radiation which does not contribute to population inversion, the laser system has no filters or absorbers.

United States Patent 91 Strozyk Oct. 16, 1973 LU:ND:YAG LASER SYSTEM ANDMATERIAL [75] Inventor: John W. Strozyk, West Long Branch, NJ.

[73] Assignee: The United States of America as represented by theSecretaryof the Army, Washington, DC.

[22] Filed: Mar. 14, 1972 [21] Appl. No.: 234,502

[52] US. Cl. 331/945, 252/301.4

[51] Int. Cl. 1101s 3/16, HOls 3/09 [58] Field of Search 331/945;252/301.4

[56] I References Cite d r 5 UNITED STATES PATENTS 3,483,481 12/1969Church at al. 331/945 OTHER PUBLICATIONS Kbstigian et al., 1st quarterlyreport contract No.

DAAB07-67-c-0562. Dec., 1967, title page and pages I9.

Primary Examiner-David Schonberg Assistant Examiner-R. J. WebsterAttorney-Harry M. Saragovitz et al.

[ 57] ABSTRACT A laser system having an elliptical laser cavity with alaser rod and a broadband pump source 'mounted in the laser cavity. Thelaser rod is a co-doped Lu:N- dzYAG crystal having a Lu/Nd atomic ratioin the range from 2.0 to 2.4 distributed in the YAG host crystal.Because the laser rod absorbs a relatively small amount of radiationwhich does not contribute to population inversion, the laser system hasno.filters or absorbers.

1 Claim, 1 Drawing Figure LU:ND:YAG LASER SYSTEM AND MATERIAL BACKGROUNDOF THE INVENTION tion may be directly or indirectly attributed to thecharacteristics of the laser materials. For example, it is well knownthat the efficiency of a laser system in inversely related to thetemperature of the laser material, i.e. when the temperature goes up theefficiency goes down. One important cause of temperature increases isdue to the absorption by the host materials of radiation which does notcontribute to the population inversion.

In order to reduce such unwanted absorptions, filters are normallymounted between the radiation source and the laser material; Thissolution has not always proved entirely satisfactory, however, becausesuch filters, while removing the unwanted radiations, also block somedesirable radiations which could be used to increase the populationinversion.

SUMMARY OF THE INVENTION One of the most frequently used opticallypumped laser materials is neodymium doped yttrium aluminum garnet(NdzYAG). Some of the characteristics of NdzYAG lasers which make thempopular are (l) the output, which is in the near infrared, is notvisible to the human eye and can be easily detected by standarddetectors, (2) these lasers can be efficiently used in either pulse orCW operation, and (3) these lasers operate as an efficient four levellaser system. However, it is generally well known that Nd:YAG lasersystems always include filtering devices in the pump cavity to removeunwanted ultraviolet radiations because of the. absorptioncharacteristics of the Nd:YAG laser materials. As explained above, asignificant amount of desirable radiation is also removed by thesefilters. If the filters were not used,the Nd:YAG laser materials wouldheat up to a point which would critically reduce the efficiency.

In order to solve this problem and to improve the efficiency of NdzYAGlasers, the present invention contemplates the co-doping of the NdzYAGcrystals with lutetium (Lu) such that the resulting Lu:Nd:YAG crystalsare rendered substantially transparent to a significant portion of theultraviolet spectrum while retaining all of the desirable features ofNdzYAG lasers. In-fact, it has been found that radiation in theultraviolet range from 3800A to 2400A actually enhances populationinversion in the Lu:Nd:YAG crystal, thereby resulting in higherefficiencies. Therefore, instead offiltering ultraviolet radiation aspresent laser systems do, the Lu:Nd:YAG laser system of the presentinvention has no filters. Actually, a pump source radiating asignificant amount of ultravioletradiation would bedesirable in thesesystems. i

It is therefore a primary object of the present invention to provide ahigh efficiency Lu:Nd:YAG laser system with a resulting reduction insize, weight and pump power normally required in NdzYAG lasers.

DESCRIPTION OF THE DRAWINGS Other objects and features of the inventionwill become apparent to those skilled in the art as the disclosure ismade in the following description of a preferred embodiment of theinvention as illustrated in the accompanying sheet of drawing whichshows a diagrammatic perspective view of a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawing, there is shown a laser system 10 having a pump cavity 11 whichincludes a re flector 12 shaped in the form of a right ellipticalcylinder. Reflector 12 may be made from any well known reflectivematerial such as gold, aluminum or silver deposited on the insidesurface of the rigid housing. Mounted colinearly with the foci of theelliptical pump cavity 11 are the laser rod 13 and the pump source 14.

The pump source 14 is a high pressure discharge krypton or xenon lampoperated at a high current density for maximum radiation brightness.There are no filters, ultraviolet or otherwise, mounted in the pumpcavity 1 1 and the reflective surfaces of reflector 12 may have a highultraviolet reflection coefficient.

The laser rod 13, besides being mounted at a focus of the pump cavity12,'is also mounted in a laser resonant cavity formed by the mirrors l7and 18 one of which is partially reflective to provide an output. Whenoperating the laser system 10, the pump source 14 is energized by anelectrical signal applied to electrodes 19 and 20 which causes the pumpsource 14 to radiate over a broad spectrum. Because of the ellipticalshape of reflector 12, almost all of the radiation which is emittedradially from source 14 will be incident upon rod 13 after beingreflected by reflector 12. Some of the radiation emitted from source 14will be incident upon rod 13 without any reflection while some more ofthe emitted radiation will be absorbed by reflector l2 and will neverreach rod 13.

The radiation incident upon rod 13 will in general be either absorbed ortransmittedby rod 13. Of theradiation absorbed by rod 13, some willcontribute to the usual population inversion required for laser actionwhile the remainder will be converted to heat. As ex-' plained above,the higher the temperature of rod 13, the less populated will be theground state andthe more populated will be the upper energy states withthe particle distribution such that the desired population inversion isreduced.

In order to improve the efficiency of such laser systems and inparticular YAG type laser systems, itbecomes important to prevent thatportion of the pump radiation which does not contribute to thepopulation inversion from being absorbed bythe laser rod In the past, asmentioned above, filters have been inserted between the pump source 14and the laser rod 13 to prevent the absorption of unwanted radiations.More specifically, in the case of the YAG type lasers, pump energy. inthe ultraviolet portion of the spectrum between 3800A and 2400A hascontributed substantially to temperature increases and the removal ofthis ultraviolet energy has been critical to the proper operation words,the rod 13 is basically a YAG crystal used as a host in which the Lu andNd are distributed and are located at the crystal sites normallyoccupied by Y. It has been found that, if the Lu/Nd ion ratio in theLuzN- dzYAG material is between 2.0 and 2.4, then, the amount ofradiation absorbed by the rod 13 which does not contribute to thepopulation inversion will be relatively small and the use of filterswill not be needed. The co-doping of the Nd:YAG crystal with Lu to theratio specified above in effect renders the crystal substantiallytransparent to radiation in the ultraviolet range. It has also beenfound that of the relatively small amount of ultraviolet radiationabsorbed by the LuzN- d:YAG rod 13, a substantial portion thereofcontributes to the population inversion.

A specific example of a Lu:Nd:YAG laser material actually used for rod13 had a Nd density of 2.05Xl atoms/cm and a Lu/Nd ratio of 2.4. A 50mmrod of this material having a diameter of 5mm exhibited a 40 percentincrease in efficiency of pulsed laser operation over an identicallyshaped Nd:YAG rod having a Nd density of 2.05Xl0 atoms/cm. Spectroscopicstudies showed substantially identical absorption characteristics forboth samples except in the ultraviolet range. The Lu:Nd:YAG sampleshowed significantly less absorption in the ultraviolet region then didthe Nd:YAG sample. Many other LuzNdzYAG samples were studied havingLu/Nd ratios ranging between 1 and 5. A significant increase inefficiency was evident for only those Lu:Nd:YAG samples having the Lu/Ndratio in the range from 2.0 to 2.4. A sharp decrease in efficiency wasobserved for samples outside this range.

It was determined that the calculated volumetric size compensation ratioLu/Nd for the LuzNdzYAG samples is 2.33. in other words, the YAG crystallattice when doped with Nd is distorted because Nd has a relativelylarge atomic radius. Such distortion produces internal stresses andstrains causing splitting of the energy levels and thereby increasingthe probability of radiation absorption over a broader band. Theco-doping of the Nd:YAG crystal lattice with Lu compensates for thisdistortion because the Lu has a relatively small atomic radius. Thedistortion or internal strain is a minimum when the Lu/Nd ratio is 2.33according to the calculations noted above. It also follows that theenergy level splitting will also be a minimum as will the probability ofradiation absorption when the Lu/Nd ion ratio is substantially 2.33 orin the range from 2.0 to 2.4 according to the results of the above notedtheoretical calculations and the more practical experimental evidence.

It should be understood, of course, that the foregoing disclosurerelates to a preferred embodiment of the invention and thatmodifications may be made therein without departing from the spirit andthe scope of the invention as set forth in the appended claims.

What is claimed is:

1. A laser system comprising:

a laser resonant cavity;

a laser rod mounted in said laser resonant cavity;

a pump source means for radiating pump energy over a broad spectrum,said broad spectrum including a substantial amount of energy extendingover the wavelength range of 3,800 A. to 2,400 A;

a pump chamber having said pump source means and said laser rod mountedtherein for directing substantially all of said radiation over saidbroad spectrum from said pump source means to said laser rod; and

said laser rod being a co-doped Lu:Nd:YAG crystal having a Lu/Nd ratioin the range of substantially 2.0 to 2.4 atoms of Lu to atoms of Nddistributed in a YAG host crystal.

